There have been many proposals as to why aliens have not contacted us but one thing appears universal in all their proposals: if these aliens exist, they are said to have superintelligence with extraordinary technology who live on planets or moons or travel to other solar systems to colonize other planets. SETI and other scientific searches focuses on extraterrestrial life. This hypothesis proposes that planets are not where we should be looking and that human-level intelligence and technology are all that's required to survive for long periods of time. But it does require the invention of non-biological consciousness.
I trust most people would agree that the best approach would be to propose a hypothesis that has a higher chance of being correct. I don't think intelligent extraterrestrial proposals stand a good chance of being correct because if an intelligent specie wants to live a long time (in the millions or billions of years), why would they live on planets, or even hang around stars where there are comets, asteroids, gamma ray bursts (GRBs), or roaming black holes that can easily destroy all life on a planet? I propose that the best chances of finding intelligent life in the universe are not around stars but in areas free of their dangers and that means somewhere deep in empty space, free of debris, radiation, stars and planets.
From what we've learned about planets in our own solar system, we know they are unstable. Earth has undergone tremendous changes from a hellish Hadean earth (4.6-4 billion years ago), to the late Archean (2.8-2.8 billion years ago) to the Early Proterozoic period (2.8-2.2 billion years ago), to the Rodinia period where Pangaea was a supercontinent, to a snowball earth (650 million years ago), to the Jurassic period when Pangaea began to break apart and the dinosaurs ruled, to the Pleistocene Ice Ages (18,000 years ago) to today.
We know there were at least five mass extinctions on earth, some caused by asteroids or comets, including the Chicxulub impact at around 66 millions years ago that caused the cretaceous-Paleogene extinction event when some three-quarters of the plant and animal species (including all non-avian dinosaurs) became extinct. If not for the Chicxulub impact and dinosaur extinction, primate evolution (and humans) probably would never have evolved. According to some biologists humans are causing the sixth mass extinction that could very well include us. They have even named it the Holocene extinction (also called the Anthropocene extinction).
We also know Mars and Venus have undergone violent changes throughout their existence. Astronomers think early Venus had oceans and may have been habitable. Mars once had oceans of liquid water and a more robust atmosphere. We know all the rocky planets and moons in our solar system contain meteor impacted craters. It's reasonable to expect most, if not all, planets in the universe have also undergone similar changes.
It's also noteworthy that about four-fifths of all the stars in the universe are binary stars. That means their Oort clouds will be churned more, thus creating more unstable planetary orbits along with the increased chance of debris collisions.
Some have proposed looking for extraterrestrial life in the center of our galaxy because there are more stars and planets there. On the contrary, there is less chance for life to evolve there because of the density of stars, especially at the galactic bulge. A planet with intelligent life would more likely survive longer at the outer edge of the galaxy where there are fewer stars. Note our sun sits about two-thirds of the way from the center of our galaxy.
As for life on earth, the average specie lifespan of invertebrates is around 11 million years and mammals at around 1 million years. The modern form of humans evolved only around 200,000 years ago. Only one intelligent species capable of written language has evolved on earth and this occurred only in the last few thousand years. Humans have only been able to use electromagnetic communication in the last 200 years and human space flight only since 1961. It appears the more advanced species have the shortest specie life spans. Human extinction appears as a very real probability (not just a possibility). So given what we know about our own species, it appears that intelligent life on a planet must be exceedingly rare.
This is not to mean that biological life is rare, however. On the contrary, it appears that the first living cells appeared at least 3.8 billion years ago, relatively shortly after the formation of our planet. We also know life can exist without sunlight from geothermal energy sources found deep in ocean bottoms. So single-cell life is likely to be abundant throughout the universe. But I am not discussing just life (which is probably abundant) but intelligent life capable of surviving solar system debris and planetary changes. It is intelligent life which is of concern here and from what we know today, it is rare.
Even if there are thousands of technological civilizations throughout a galaxy, as some speculate, that doesn't mean they are all existing at the same time. A one-million year average lifespan of a specie (and probably much lower for a technological specie) would rise and fall in a flash of time compared to the age of the universe. It would be like trying to photogragh the flash of a firefly through a long narrow tube. For these reasons, I suspect the chances of ever detecting a technological extraterrestrial species are much rarer than most people think.
Also, consider the social complications involved in a specie that has managed to evolve the capability to do science. What if the specie members reject evolution or scientific thinking?
False beliefs are a poor way to survive for long lengths of time
Even if a specie could survive on a planet long enough to develop intelligent life, consider that a planet has only a limited amount of space and natural resources. This virtually guarantees conflict, not only between species, but within species. If intelligent life evolves elsewhere in the universe it likely also developed fight & flight instincts, and in social species with language, emotions and beliefs. And once you have beliefs, you also have the probability of beliefs without evidence: faith. A species that relies on faith virtually guarantees a short specie lifespan.
One of the threats to our civilization comes from people who deny science and want to replace it with delusional beliefs. We see this today from religious, political, and ideological extremists who are more than willing to destroy all human life on earth if they ever get their hands on weapons of mass destruction, or genetic engineering which could result in creating a virus that could wipe out our entire specie, or corporate greed that cares for itself rather than life of earth. Nor does it even require destruction of a specie. It only requires the reduction of technology. Similar to the European Dark Ages that delayed technological progress for over a thousand years, we could end up in a post apocalyptic age as described in many science fiction films such as Mad Max. Here's what we face: Nuclear or biological warfare, pandemics involving one or more created viruses, antibiotic-resistant bacteria, climate change, death to the oceans and the extinction of fish, loss of fresh water and food (due to mass extinctions).
The Drake equation (a probabilistic argument used to estimate of the number of communicative extraterrestrials in our galaxy) includes the lifetime of the "communicative phase" (the "L" in the equation above). Even if a specie has the communications technology available to them, that doesn't mean they will use it if their belief systems disallow it, even if that belief comes from secular sources. For example, if a specie believes aliens are hostile to planetary life (without any evidence), then they might decide to hide their technology or not send out any signals into space at all. In fact, here on earth there are many debates about this. For just three examples, see: [source 1, source 2, source 3]
Science doesn't require beliefs. As Lawrence Krauss said, "As a scientist, I don't believe anything. Science shouldn't use the word belief. There are things more likely and less likely. Science can say nothing with absolute certainty." If an intelligent alien civilization is able to survive and send out signals for millions or billions of years they will most likely have tamed or eliminated beliefs from their world view and adhere to scientific methods of obtaining knowledge.
The problem with alien terrestrial colonization
Theorists claim that if an intelligent alien's planet becomes too dangerous to live on, the aliens will search other livable worlds and colonize planets in other solar systems. This makes little sense. Consider that colonizing other planets involves traveling light years of distance. If you can survive that long in space, why in the world would you need or even want to live on a planet? Why not simply remain in space where you've adapted? Consider that humans evolved from ancient vertebrate fish millions of years ago and adapted to live on land. Similarly, a terrestrial species that adapted to live in space is more likely to remain in their adapted space. Moreover, living on a planet means fighting gravity along with all the other life threatening dangers (as described above) along with earthquakes, volcanoes, storms, limited resources, limited space, and all the social conflicts that can arise. Planets have finite areas, thus guaranteeing death of its specie members, otherwise the planet would soon become overpopulated. If a specie wishes to overcome death, space affords them the unlimited volume to reproduce without fear of dying or extinction.
The only reason for wanting to colonize other planets is if the aliens are biological. However, this is unrealistic. Biological species need an atmosphere to breath, biological life to eat, and gravity to prevent bone loss and other degenerative diseases. Spending years in space would require bringing along a pressurized atmosphere, food and water, medicine, and the problems of replenishing them. There's also the problem of high-energy, ionizing cosmic ray nuclei (HZE) that affects biological life, especially damage to cells, DNA, and the immune system over long periods of time. Loss of gravity also takes a toll on a specie evolved on a gravitational planet. The American astronaut, Scot Kelly, for example, spent only one year in space and he suffered bone loss, muscle atrophy, heart shrinkage, decreased vision, and accumulated a years worth of radiation from space. There's also the problem of psychological stress and adapting to space. Of course it might be possible to survive in suspended animation, but so far no one has figured out how to do this, especially considering the vast amount of time involved in keeping a body in hibernation without destruction of cells. If a specie were to survive long trips through space, they would have to adapt to the conditions of space, and that means designing their bodies to live in space.
It only had to happen once
What I have been describing is actually an extension of the Great Filter theory, first proposed by Robin Hanson to solve the Fermi Paradox by explaining that there are barriers that prevent civilizations from living long lives. This is a valid concern, but all the great filter proposals apply only to planetary or star based civilizations. None of the filters apply to intelligent beings who have adapted to live in space away from star systems. Indeed, intelligent terrestrial life is improbable, but even if one terrestrial civilization was able to adapt to living in space, intelligent life throughout the universe suddenly becomes very probable. Even self-replicating spacecraft theories, von Neumann probes, and replicating seeder ships are always connected to planetary solar systems. All these proposals are doomed by the Great Filter. Just as the first reproducible life on earth must have been exceedingly rare, it only had to happen once for natural selection to take hold to produce the millions of species that we see on earth. Similarly, regardless of the rarity of an intelligent species evolving on a terrestrial planet, if even only one specie was capable of surviving in space without the need for planetary support, it could theoretically spread throughout a galaxy, possibly evolving into many different species.
We should escape the anthropomorphic worship of stars and planets. Planets and stars should be considered dangerous not desirous. Using the technology we have now allows spaceships to travel in space without the need of stars.
To live for long periods of time, you cannot rely on the statistics of a short lived specie. As Neil deGrasse Tyson said, "The Universe wants to kill us". A specie that can survive for millions of years would have to avoid all the things in the universe that could kill them.
The science fiction writer, Robert A. Heinlein, created a fictional character, Lazarus Long, who lived an unusually long life of well over two-thousand years. He had to avoid common human activities such as joining an army because doing so would dramatically increase the chance of dying in a war. Even such benign things like driving a car would would virtually guarantee your demise due to the statistical increased chances of dying in a car accident.
For example, if your chances of getting hit by lightning in one year are 1 in a million, those odds drop to 1 in 12,000 over a lifetime. If you want to live 12,000 lifetimes, your chances of getting hit by lighting is near certain, so you would have to avoid places where lightning hits. Time turns the possible into the probable.
A similar kind of thinking would have to apply to an alien specie. If you want to "live long and prosper," solar systems are not good places to hang around.
How would a specie evolve to live in space?
One proposal to surviving space is to genetically modify the human body to prevent damage from cosmic rays, bone loss, etc. For example, it might be possible to modify skin cells to absorb ion radiation, bones that do not lose calcium, etc. Regardless of how much is done to modify a body, it would still require food, air, and water. A better solution would be to design a body capable of utilizing the resources already available in space rather than the resources from a planet, and that means getting rid of the necessity of food, air, and water. Electron powered non-biological life solves the problem. Its "food" would consist of electronic energy stored in batteries, gathered from solar panels, produced by generators powered by hydrogen cells or whatever future power sources they might invent. Electrons are everywhere and cheap, food is neither.
We know Earth's natural resources are becoming scarce, and rare-earth elements are becoming more desirable as our technology advances. For example, scandium is used in metal alloys used for aerospace components, neodymium is used in magnets, lasers, and capacitors, etc. Other metals such as lithium (used in batteries) are found in only a few countries and of course, gold, silver, and platinum are highly desired in many electronic components. The only other place to find these resources are asteroids and comets. So it's inevitable at some point in our future, we will have to mine asteroids and comets if we wish to continue and advance our technologies.
There is now an interest in asteroid mining for these raw materials including water and oxygen to sustain astronauts; as well as hydrogen, ammonia, and oxygen for use as rocket propellant. At the time of this writing there are two companies devoted to developing asteroid mining, Planetary Resources, and Deep Space Industries. In fact asteroid minding may become a reality as soon as 2025! The first mining operations will be done by astronauts and, eventually, by intelligent mechanical robots. This is not as far fetched as it might seem. DARPA is already undergoing robot engineering with the hope that they can be used for satellite repair missions. And if they can be used for repairing satellites, they could certainly be designed for mining asteroids. NASA is also undergoing a robotic mining competition for university-level students to design and build a mining robot that can traverse a Martian terrain.
These will be intelligent robots with the ability to communicate with humans. We already have intelligent systems that can not only understand human language, but can out perform humans in many areas such as in chess (IBM's Deep Blue that beat the best grand master), the game of Go (Google's DeepMind) that beat the best Go player in the world, and in general knowledge where the best Jeopardy champion was beaten by IBM's Watson. We already have autonomous robots such as aircraft drones, deep sea mining robots, autonomous cars, robotic spacecraft, and planet rovers. Similar intelligent systems will be designed into future mining robots, and they will be more capable than human miners.
The next step in asteroid mining will be in making the operations more efficient and profitable. It's expensive sending robots and mining equipment into space, not to mention the repairs that will have to be done to them. There will be a desire to manufacture robots in space rather than on earth. And who will do the manufacturing work and repairing? The robots themselves of course. And once this is achieved, the robots become self-replicating (note, engineers have already built simple self replicating robots, and we have the beginning of utilizing 3D printers for manufacturing in space).
Even though these first mining robots will be intelligent, a small number of humans would still be needed to control them. They would need a human commander to give them orders such as how much to mine, how many robots to manufacture and so forth to reflect the desires of their corporations back on earth.
Because of the problems of humans living in space (as described above), the human crews would have to be replaced by new crews every year or so (or shorter). That means mining only near earth orbit asteroids. There will be a desire to mine asteroids further out, perhaps into the asteriod belt, but because this is so far away from earth, communication delays due to the light speed limit could create problems if they need quick decisions from earth. Humans can make these quick decisions if emergencies arise and to create solutions to problems. But this is too far to practically send humans. The mining corporations will want to replace human astronauts with artificial sentient systems to avoid these problems. And this presents a problem.
But they are not alive!
Even if we produce intelligent self-replicating robots, that doesn't mean they will be sentient. Intelligence doesn't require consciousness. To have sentience you must have qualia (feelings and emotions). Some philosophers might dispute this such as Daniel Dennett, but although there has been great progress in intelligent design, there has been zero progress in creating a computer that can feel. And this is a problem if you want sentient life to spread throughout the universe.
Regardless of how much intelligence a robot has, they will behave like zombies without curiosity or a will to live. There is no reason to expect them to spread throughout the universe, much less try to communicate with other intelligent species. I suspect they would behave like the automata in a super Game of Life and eventually die out. A species that survives would need desires, the will to live, and especially the curiosity and wonder to explore, invent, and to create. This requires feelings and emotions, the driver of consciousness.
Once AI scientists are able to engineer sentient circuits equivalent to brain circuits such as the nucleus accumbens (responsible for reward feelings), the cingulate cortex (responsible for emotion formation) and other sentient circuits, along with the equivalent of hormones and neurotransmitters, then these could be integrated into artificial intelligent systems (with time to learn to adapt to these new circuits) to create a fully sentient specie. We will have have invented, not just a new species, but a new kingdom of life that dropped its reliance on DNA and got rid of all diseases, parasites, viruses, and bacteria and they will no longer need to kill living things to keep themselves alive. Their life spans would be unlimited and they won't need to wake up in the middle of the night to pee!
An average human brain has around 86 billion neurons and each neuron has thousands of synapses. An electronic brain's artificial synapses would need to match the number of of synapses in a human brain. The first synthetic brains would be too huge to put into robots. Besides, there's no need to give robots consciousness because, why risk sentience in risky mining operations? Instead, imagine a conscious artificial brain controlling the robots from within a spacecraft. The spaceship itself, would serve as a conscious "body" and its robots as the workers and maintenance crews. Note we humans have our own workers and maintenance crews installed into our bodies doing immune maintenance work, digesting food, etc. They are the beneficial microbiota in our bodies that outnumber our human cells.
It is these space mining spaceships with robot miners and artificial sentient commanders that would serve as the best candidates for space explorations beyond our solar system. Imagine a mission consisting of a fleet of spacecraft along with mining ships and manufacturing factories, being sent into deep space. And once that happens, the genesis of alien life could take hold throughout space.
Electronic brains would benefit from living in the cold environment of space because they could take advantage of superconductivity making their brains very energy efficient. The temperature of objects in empty space between stars approach absolute zero, somewhere just above -270 degrees Celsius and well below what is required for superconductivity.
There's nothing new about proposing biological life evolving into non-biological bodies. Science fiction authors, philosophers and scientists like Paul Davies, Steven J. Dick, Ray Kurzweil, Nick Bostrom and many others have made such suggestions. The philosopher Susan Schneider wrote a piece called "Alien Minds" that does a good job describing the problems of non-biological aliens, with and without consciousness. However, all of these prognosticators propose that aliens would be super intelligent. Although that may very well be the case, it is not a requirement. A specie only needs human-level intelligence to survive. What does superintelligence even mean anyway, and how do we know there aren't limits to intelligence? No doubt aliens would be more knowledgeable than humans but knowledge is not the same thing as intelligence. And as Dennett has reminded us, human brains, the most complicated thing we know of, were created by very low intelligent robots (biological cells) through natural selection. I propose a minimal hypothesis that doesn't require superintelligence, just human-like intelligence because this is more likely to be achieved than superintelligence. But creating intelligence with consciousness that includes feelings and emotions? That's another matter.
Unfortunately consciousness is the most problematic part of this hypothesis. Although we have good ideas about what consciousness is not, we have no good theory about how to build a conscious machine (intelligence, yes, but not consciousness). Nevertheless, there are several programs that are attempting to build artificial brains that would mimic human brains.
Even if we are successful, we may decide not to give computers consciousness simply because of the ethical questions and all the consequences that go with it. So everything I propose in the rest of this hypotheses rests on the speculation that this is possible and that at least one extraterrestrial specie has figured out how to do it.
If any specie has done it, these sentient artificial life forms would evolve via natural selection by using memes. Artificial sentient beings will not only have the capability of inventing spaceships, they would be able to design themselves, advancing their own species or inventing an entire line of species. They might even be able to tweak their own minds to control their emotions or to shut them off entirely depending on the situation they are in. There are so many possibilities, one could write endlessly about them. I will describe only minimal obvious choices that aliens would make that might give clues as to where we might find them (and it won't be on planets).
How will they travel in space?
A major concern would be fuel and the first fuel might be hydrogen-oxygen based like that proposed by Planetary Resources. Hydrogen is the most abundant element in the universe and oxygen the third most abundant. Both these elements can be extracted from ice on comets and asteroids. Uranium, and other radioactive elements (found in asteroids) could be used to power nuclear fission reactors. Humans are already on the brink of solving the fusion power problem and no-doubt that other intelligent life-forms will have developed fusion reactors. Because fusion energy works like fusion in a star, their space crafts could very well carry on board a mini-star to power their crafts. Actual starships. Once a specie develops efficient fission or fusion reactors in space, there's no need to build large and expensive Dyson spheres or solar arrays around stars, especially if you wish to avoid dangerous debris that surround stars. Note we already have nuclear powered space probes. Later, more efficient means such as using star light to power very efficient super cooled electronics could serve to power everything on board their starships, especially while traveling long distances between stars. Moreover, aliens would be able to utilize gravitational sling-shot techniques to steer and travel through space with very little power usage (similar to the way we already slingshot our space probes through the solar system). Even low-tech steam power could be used for propulsion.
Duty Cycle spacetime travel
Once aliens (or us) are able to design a living electronic brain, they will be able to shut it off and turn it on again at will. This low-tech ability alone allows a revolutionary and exciting form of space travel: spacetime travel utilizing electronically controlled suspended animation. Aliens could change the perception of time and travel instantly from one point to another even if that distance is thousands of light years distant. This method requires no new theory of propulsion, wormholes, superintelligence or any other unproven theory of time travel and does not contradict Einstein's general theory of relativity. How so? By utilizing electronic duty cycles coupled with a spaceship traveling at sub-light speeds in spaces free of debris (between stars and galaxies).
The following description comes from Death and Time Traveling:
The term duty cycle comes from electrical engineering terminology. A duty cycle represents a periodic ratio of on and off states. Usually duty cycles are represented as square waves or rectangular pulses. If you've ever operated a microwave oven at 50% power, or adjusted the thermostat on your air-conditioner, you'd realize they operate by turning on for a period of time and off for a period of time. Microwaves, stepper motors, power drills, as well as many computer controlled devices use duty cycle power controls. An example graph of a 50% duty cycle showing five on-states looks like the following:
Now that you understand duty cycles, imagine an alien pilot controlling its own duty cycles to match the aims of its voyage. Since it utilizes electrical circuits it can switch its "brain" on and off almost instantly. The on-state period of duty cycles represent the conscious states and the off-states as time suspended states (these "off" states are actually temporary death periods). By using memristor-like circuits, the off-states would automatically preserve memory while the on-states would instantly return to conscious operation without boot-up times. Note the synapses of the brain are, in effect, memristors. Memristors also have the advantage of long life memory storage. Because they are made out of titanium dioxide their life is in geological time spans, at least 100 million years or longer, and if they are in near-absolute zero temperatures of space, they could last for billions of years, ideal for a specie that wants to live for long periods of time.
Important to this concept, you should understand that an off-state duty cycle could remain off for extremely long periods of time, perhaps hundreds or thousands of earth years time, while each on-state may last only a second or a fraction of a second. Note, since the off-states are also death states, the aliens will not fear death because they will "experience" death every time their brains are turned off. Since only the on-states are perceived, conscious time would appear seamless to the time traveler. The duty cycles from the perspective of an outside observer might look like this:
One light year represents the distance traveled at light-speed in a vacuum for a period of one year. This calculates to a distance of approximately 5.88 trillion miles. But to the space voyager, the conscious period would feel continuous and he would have no awareness of the off-states. Our spacetime traveler would experience a period of 5 seconds of consciousness like this:
So in 5 seconds our alien would have traveled a distance of 4 light years or over 23 trillion miles!
In these illustrations I have given each conscious state a period of one second. Actually it would more likely fall far shorter. A human brain processes visual stimuli in about 50 to 100 milliseconds. A computer can switch in microseconds. Since it would power on for only a few milliseconds every light year or so, the power consumption should prove exceedingly low.
If you still have a problem understanding this, imagine a camera in a spaceship taking a snapshot of the universe at a distance of every light year or so. After you've taken a thousand snapshots (covering 1,000 light years) and assembled each snapshot into a movie film, you'd have about 30 seconds of film showing a voyage through the universe (motion-picture frames display between 24 and 30 frames per second). If an alien set its duty cycle at this rate, it would take only a few days (from its time frame) to reach the other side of the galaxy! Of course our alien traveler could get there instantly if he set the duty cycle to one cycle (off when you begin the journey and on when you arrive).
A spacetime traveler could also accelerate and decelerate by adjusting the duty cycle to match "throttle" and "brake" inputs:
With duty cycle time traveling, the experience of traveling through space becomes dramatically transformed. Imagine an alien starship pilot controlling a duty-cycle "throttle" that can accelerate and decelerate similar to flying an airplane over a landscape.
If you've ever watched Star-Trek movies, you know the virtual feeling of flying past the stars. Our duty-cycle time traveler should experience a similar kind of exhilaration like this:
As an added bonus, since these duty-cycle slices through time would be experienced in digital electronic brains, they could record each slice. This would afford them the ability to travel backwards in time, in Tivo-like fashion, and they could replay their history exactly as they experienced it the first time. This would also allow them to share their experiences with other aliens (the sci-fi movie, Strange Days, explores recordable consciousness).
Aliens may wish to transmit these signals via electromagnetic radiation. The information in these signals may contain, language, audio, video, or a combination of these, along with conscious states in a broadband spectrum. So here we have three ways of experiencing consciousness: directly from brain circuit propagation, replayed from stored memories, or transmitted electromagnetically to other brain "receivers."
Needless to say, reality for them would take on new meanings. And there is nothing technologically unfeasible about this at all, as long as you have the ability to create digital electronic consciousness.
The down side of this kind of space travel, but only from an earth based perspective, comes with the fact that planetary civilizations would rise and fall within a few duty cycles of the journey. The term "home" would have no meaning for such a being except in the context of the starship itself. This might be one of the reasons why we have never detected alien signals. Since our space traveling alien controls its perception of time, it exists when and where it wants. Humans control three dimensions, but space aliens could control the perception of the fourth dimension-- time. They would truly be four-dimensional beings.
This does not mean aliens would live by themselves without the need to communicate with other aliens. An alien would have a difficult time surviving alone in space, especially considering the unknowns it would encounter. It would appear no more reasonable to expect a single alien to survive anymore than a single member of a mammal specie surviving on earth. Aliens should have similar qualities of all known intelligent animals-- a social structure. The astronomer Stephen Dick wrote about the possibility of alien culture. For example, mammals live within some social group to help them survive so the same might apply to our space aliens. Social aliens would have an advantage for survival because they could cover more volume and accumulate and share more data and knowledge of the cosmos. But to convey this information to each other requires a way to communicate.
The most obvious method of transmission would consist of the fastest known kind of communication: electromagnetic radiation. Aliens might use radio, microwave, light, or perhaps gamma rays. But even at light-speed velocities, they would have a problem with communicating unless they stuck close together. Consider that it takes 10 minutes for signals to reach Jupiter from Earth, imagine the years it would take to reach light-year distances. They would need a faster way to communicate. Again, duty cycles come to the rescue. Duty cycle communication. Faster than light. Instantaneous.
Consider two space aliens separated by a distance of 25,000 light years. Each alien transmits signals to match each other's duty cycles, waiting for the signals to arrive during duty cycle off-states. When the signal arrives, their brains turn "on" and they will consciously perceive it. To the aliens, their communications would appear fluid and instant. To resolve time delay problems, it would be no more difficult than what the internet does today when networks routinely use protocols to re-route and receive time delayed packets of information through a network.
Protection from the dangers of traveling through space during "off" duty cycle states
The first explorers that travel away from their home star would have to travel through their solar system's Oort cloud. This presents a problem because of the dangers of high energy comic rays, comets, and asteroids. Even electronic sentient beings, and sensitive electronic circuits would have to be protected from cosmic rays and gamma radiation. Various proposals have been made for protection. A dense material such as lead could absorb X-rays and gamma radiation while a water-ice shield could be used to absorb cosmic rays. H2O is a known good absorber of cosmic rays. An alternate method could use a magnetic shield, perhaps powered by superconducting materials. Remember that space is near absolute zero temperature, ideal for using low temperature superconducting materials, and it would require very little energy to power them. As for the dangers of comets and asteroids, long range radar could be used to detect them, giving them time to course correct away from them. Note, we already have radars that can detect asteroids and comets. Although, the sentient spacecraft would be "asleep" during "off" duty cycle states, low powered sensors could monitor the ship(s) during long distant travel. If a problem is detected, maintenance robots could repair any damage that might occur. If an emergency occurs, the sentient part of the ship could be awakened to take care of the problem. Remember also that the ships would have factories available to them to make any part, robot, or even an entire ship, if the need arises. The raw materials could be gotten from asteroids and comets. If the worst happens, and a ship is destroyed by some unknown means, there would be other ships to carry on. The ships in the fleet might be spread apart by thousands of miles or so to increase probability of survival. Once the fleet gets past the Oort cloud the dangers should be greatly reduced. When they enter low debris-free space, they could theoretically survive for multiple millions or even billions of years.
How will they live in empty space? There's nothing there!
It's reasonable to expect alien species to fill available safe niches in space similar to the way animals fill their terrestrial niches. Like early Americans who explored the wild west, some of them formed towns and farming communities, while others went on expeditions like Lewis and Clark, and others went on hunting trips. So too might the aliens form close-knit communities in space while others explore the cosmos hunting for answers in the universe (alien scientists).
You might wonder: If aliens are to live eons in space free of debris, how in the world will they survive without replenishing their resources? Once alien species are able to find safe areas to live, they would no longer need their risk their lives in mining areas. So an obvious solution would be to send autonomous robots to do the mundane risky jobs. They would send their mining robots to and from Oort clouds or supernova explosions (where hydrogen and the heavy elements exist) to gather elements and transport them back to the space communities. These "slave" robots might sacrifice themselves to solar system debris every now and then but their owners would remain safe at their deep-space homes (similar to an ant colony). Imagine a robot shuttle service between an Oort cloud and alien homes. These mining robots would certainly communicate to each other but I see no good reason why they would send signals to terrestrial planets. Because asteroids and comets are also in Oort clouds, they wouldn't need to travel into inner solar systems or anywhere near habitable goldilocks zones, providing another reason why terrestrials would not detect them.
Because robots can mine all the elements they need from space the aliens would have no need for planets, thus no reason to threaten terrestrial life. Why fight gravity when you can mine low gravity asteroids? Why would any alien specie risk their lives visiting earthlings, who's dominant specie consists of religiously militant primates living on an overpopulated dying planet?
Unlike finite planet surfaces, space is so vast and offers unlimited resources, the aliens would have no need for war or to conquer terrestrial civilizations. Stephen Hawking disagrees with this as he thinks there's a risk claiming that, "If aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America, which didn't turn out very well for the Native Americans.” I answered this in An argument against Hawking's hostile extraterrestrials hypothesis.
Why live in space, the aliens would be bored!
Since empty space has nothing in it, what in the universe will aliens do? Won't it be crushingly boring? Not at all. Intelligent aliens could construct virtual realities and knowledge centers. In fact I would expect they would spend more time in virtual space than in universal space because there are more opportunities for discovery and exploration there. The material universe contains limits such as quantum Planck scales, light-speed limits and the constraints of universal constants. Living in a mathematical virtual reality has an infinite number of possibilities (see Cantor infinities). Moreover, they will no doubt be designing new sentient circuits where they will be able to improve aesthetically pleasing sensations as well as creating entirely new sensations. Art and beauty will not die in this kind environment.
Note that humans have only begun to explore virtual reality. The first commercial virtual headsets will be arriving the year this article was written, and some companies expect it to soon dominate the entertainment market. Already we have laser-scanned mapped territories such as auto-racing circuits used in racing simulators. A million year old alien civilization should have the ability to send robots to laser-scan entire planet surfaces with resolutions greater than human vision and construct virtual worlds to live on them if they wanted to (much safer than living on real planets). They could also relive their recorded past, or invent new geometries, explore Banach spaces, or create new dimensions in Hilbert spaces. The possibilities are endless. There's no reason why they should become bored.
If you wish to survive as long as you can, it's best to live in the safest places in the universe. It stands to reason then that those who enjoy and respect life the most and wish to live the longest will want to migrate to the safe spaces. The tendency will be to live in the most popular, happiest and safest of these civilizations. What would the environments look like to hold these civilizations? Perhaps cities in space, spread out over millions of miles. This would give them an advantage of specie survival because even if a city were destroyed by a gamma-ray burst (or some other matter destroying event), other cities would survive, thus guaranteeing specie survival. This is far more advantageous than living on a planet where all the cities could be destroyed by a single catastrophic event.
Why aliens may not be looking for terrestrial life
As long as aliens possess curiosity, they would, no doubt, be interested in discovering new intelligent life, but I would think they'd be more interested in looking for other alien space species like themselves rather than terrestrial creatures like us. Such alien species would understand science (a requirement for living in space) and they probably thoroughly understand how biological evolution works. Even if some of their scientists want to search for terrestrial life, there will probably not be very many alien scientists devoted to this kind of research for similar reasons as human biologists studying species on earth. For example, there are over 7 billion humans on earth but how many biologists are actively looking for new species? Not many and when they do find a new specie, it's usually by chance. Moreover, with billions of stars in a galaxy and probably trillions of planets, the odds of finding us would be remote at best.
There's a tendency for all matter and energy in the universe to evolve toward a state of inert uniformity. This is one definition of entropy. Eventually everything degrades and falls apart. Long lived aliens would also have to be concerned with entropy. Since synthetic aliens are self-built, this affords them the ability to simply replace bad parts and circuits whenever needed. Even if aliens have a shorter than expected life spans (due to damage from some unknown means) they could simply reproduce more of themselves. After all, this is how biological species are able to last for millions of years. But even here, a specie would eventually succumb to universal entropy when, according to some scientists, heat death of the universe will cause the stars to eventually burn out, and all matter decays into energy. This would end all life in the universe.
Where will we find aliens?
I submit that it is more probable that aliens are living in space rather than on planets for the reasons given above. So where in space might we find them? This is pure speculation but perhaps they live between stars in the areas most free of debris, or at the upper edge of a galaxy but still close enough for their mining robots to reenter the galaxy to gather elements from Oort clouds, nebula gas clouds, supernova and colliding neutron star explosions. Physicists now think neutron star collisions are responsible for most of certain heavy elements such as as gold. Since neutron star collisions are extremely rare, this might be a prime candidate to find alien mining robots. Or maybe they are orbiting ancient isolated black holes where they had long ago cleared the area of debris with the added advantage of using its powerful gravitational fields to slingshot mining robots or space explorers to their destinations. I would expect only a few brave sentient alien adventurers to risk their lives in solar systems.
The last remaining dangers from debris would come from the spaces between galaxies where rogue stars, planets and asteroids, and perhaps mini-black holes roam about at high velocities, thrown out by gravitational forces when galaxies and black holes collide. So to protect themselves from rogue debris, perhaps alien colonies have developed a cloud network of robotic "guardian sentries" surrounding the colonies. These sentries could detect dangerous objects approaching, then signal the colonies so they can move safely out of the way. Since these close-knit colonies in space would not need to duty-cycle their communications, we might be able detect their signals. Moreover, navigating space would be difficult so perhaps the aliens have deployed navigation buoys in debris free space similar to the way channel buoys steer ships through safe waters. These buoys might be sending out powerful electromagnetic signals so the traveling aliens have a way to easily detect them.
However, it might be extremely difficult to detect any signals coming from them because their communications might be laser focused toward each other and their mining robots. The aliens that do travel through space are probably duty-cycled in hundreds or thousands of earth year increments. Perhaps the famous "Wow!" signal detected by SETI in 1977 might have been one of these duty-cycled signals.
I think I have given a more likely hypothesis for aliens living in space rather than superintelligent aliens living in solar systems. This is a minimal hypothesis that only requires one yet unproven idea: creating conscious non-biological machines. Everything else relies only on technologies that humans already have or are, in principle, doable in the near future (such as asteroid mining). It only requires human-like intelligence, not superintelligence. It does not require extraordinary technology such as Kardashev civilizations, Dyson spheres, megastructures, wormholes, spaceships traveling close to the speed of light, suspended biological animation, or any unproven scientific hypothesis other than non-biological consciousness. Great Filter theories apply only to terrestrial life not to aliens evolved to live in space. If a hypothesis has a possibility of being correct then, if given enough time, that hypothesis will eventually happen. Since we don't know if it's possible to make a non-biological conscious mind, I give this hypothesis a 50% chance of being correct. But if we ever figure out how to design artificial consciousness, then I give the odds in the 90-99% range that at least one intelligent alien specie has figured it out too.
If the ideas presented here have actually been carried out by only one terrestrial alien civilization in our galaxy they could, in principle, be evolving and living in space and we wouldn't be able to detect them because we are looking in the wrong places.
So where are the intelligent aliens? There's probably no paradox. They are in space, and only rarely on planets (because of short terrestrial specie life spans, and the dangers of unstable planets). They probably haven't contacted us because biological beings are not their main focus of interest. Space traveling aliens would use duty-cycle spacetime travel and they would be difficult to detect because their "off" states would be much longer than their "on" states. During their "on" states, they would probably spend more time in virtual worlds, communicating with themselves, mining elements, or looking for other space aliens like themselves. If we have any chance of finding them it's more probable to find them in debris free space. They will appear as Lazarus Longs to us, evolved from computers invented by intelligent biological beings who once lived on a lonely dangerous planet that no longer exists.
Note: Since no one knows what alien space ships would look like, we only have imagination to work with. So for a little fun, I created a little slideshow of spacecraft images, gathered from the internet. To keep with the hypothesis proposed, I chose images without planets or close stars in the background. Also no war craft or ships designed for humans or biological humanoids. All these space ships look like they could be built with human-level intelligence (not superintelligence). Because non-biological aliens would live in electronic circuits, they could be installed anywhere: robots, starships, or in data centers in deep space colonies. Click here to start slideshow.